The present invention relates to a method to lower the formation of clods and the clustering tendency of reducible iron-containing agglomerated material, particularly so-called pellets.
Extracting metallic iron takes place, among other means, by the direct reduction of iron oxide in a direct reduction furnace according to the so-called DR process, whereby the iron oxide, commonly in an agglomerated and concentrated form, preferably the form of so-called pellets, continuously moves down through the shaft of the furnace during the so-called charging of the furnace where it meets a reducing gas with a temperature of about 800-950.degree. C. The reducing gas reduces the iron oxide so that metallic iron, the so-called iron sponge, is obtained, which can then be fed out in the lower part of the furnace. Since the reactivity of the iron oxide increases with increased temperature, a raised reduction temperature is generally desirable as this leads to a quicker reduction process and thus increased speed of production.
The disadvantage of a higher reduction temperature is that the iron-containing reducible agglomerates in the furnace show a greater tendency to form clods and stick together and form ever larger agglomerates, so-called clusters. These clusters slow down the reduction process since they hinder the flow of gas and materials, which in turn leads to a lower speed of production and a final product with a comparatively lower quality.
In an effort to increase the production yield or production value during the reduction process today, an increasingly larger amount of reducible raw material of very high purity, i.e. raw material preferably with very low quantities of silicon, is being used. However, the high iron content of the reducible material also contributes to increased formation of clods and a tendency to cluster in the reducible material.
There is thus a desire to reduce the clod formation and the tendency to cluster in the reducible iron-containing raw material that is used in the reduction process, and consequently even the possibility to raise the temperature at which the reduction process takes place. Experience from operating processes has shown that increasing the reduction temperature during direct reduction by 100.degree. C. allows an increase in the speed of production of the iron sponge by 25%, at the same time as the specific consumption of gas can be held essentially constant.
Until now, the commonest solution to the problem of cluster formation during the direct reduction of oxidizing iron-containing material has been to lower the temperature of the reduction process, which is not an acceptable solution from a production point of view. Another way to avoid clusters is to reduce the iron content of the reducible agglomerates. However, this is not an acceptable solution either as it also leads to a lower production yield during the reduction process and the production of iron.
Within the technology, it is also well known to solve the problem by so-called "coating", i.e. coating the outside of the reducible iron-containing agglomerates with a protective layer of a non-iron material, a so-called coating material, that hinders the agglomerates from coming into metallic contact with one another during the reduction process. These non-iron materials should have a small particle size so that they easily adhere and completely or at least mostly cover the surface of the agglomerates, and have a melting temperature that exceeds the temperatures found during the reduction process by a considerable margin. The said non-iron materials commonly consist of limestone, lime, dolomite, and thus of material that does not harden on contact with water, and that in particle form and when forming part of a liquid suspension, a so-called slurry, is applied to the iron-containing agglomerates as a final step in their chain of manufacture.
As experience has shown that all handling of the coated agglomerates up to the charging of the furnace for reduction affects the degree of coverage of the applied coat in a negative manner, and that this should therefore be avoided, the coating of the iron-containing agglomerates has so far been carried out as late as possible in the chain of manufacture, and in such a way that the number of points of contact and the internal movement of the agglomerates relative to one another is restricted as far as possible.
In an effort to achieve this today, the coating is usually applied as the final step in the chain of manufacture of the reducible agglomerates, i.e. after the sintering and the following desulphurisation of the agglomerate. So far, the application of a coating on the agglomerates has, for example, been carried out by spraying and hosing with a coating material in slurry form directly onto the agglomerate while this is located on a conveyor, such as an endless conveyor belt or similar. Studies have shown that with this technique, only about 1/3 of the agglomerates acquire what can be considered to be an acceptable degree of coating. Another technique for coating agglomerates that has recently be practised to an increasing extent is the utilization of a drop or descending shaft, whereby the coating is applied by spraying or hosing with a coating material in slurry form from different directions while the said agglomerates fall freely through the air. Even if these means to a large extent avoid the particles coming into unnecessary contact with one another during and after the coating, the method has not been shown to provide the even and continuous overall coating of the agglomerates that is normally needed to prevent clustering in the reduction process. In particular, this is probably due to the falling agglomerates wholly or partially covering one another during the spraying process. Those attempts that have so far been made to solve this problem have mainly been focused on increasing the amount of space between the agglomerates and their distribution during the actual spraying process, in combination with increasing the height of the fall and arranging further hosing nozzles along the moving flow path of the agglomerates. However, these measures have not produced the sought after results with regard to the degree of coverage of the coating. In practice, it has been shown that dust and particles given off by the free-falling agglomerates tend to block the hosing nozzles, in particular those nozzles the lower part of the flow path and that are thus located closest to the landing place of the agglomerates.
There has long existed a desire to be able to improve the degree of coverage and quality when coating iron-containing agglomerates with fluxing material and maintain a high speed of production, and the objective of the present invention is to achieve a method that realises this desire. This objective of the invention is realised by it having the features and characteristics specified in the claims.
The use of a drum to apply a coating of a fluxing substance to an agglomerate is certainly known from U.S. Pat. No. 3,975,182, but it should be pointed out that even though the use of a drum is mentioned in this document, the intention of using it is not as in the present invention, namely as a final manufacturing step to as quickly and effectively as possible apply a protective fluxing substance in the form of a liquid suspension, a so-called slurry, to a formed reducible agglomerate, but to utilize a traditional rotating procedure in a rotating drum to apply a layer of fluxing substance on a so-called green body. Regarding this, it can be mentioned that the formation of so-called green bodies generally takes place when primary cores of particulate materials are, like snow balls, rolled-up layer-by-layer in a rotating drum that has a bed of moist, fine grain iron-containing material and where growth takes place as a result of the effect of adhesion. According to the said document, the same principle is used to apply the green body formed with one or more layers of fluxing substance, for example, limestone, by rotation in a complementary rotating drum that has a bed of moist fluxing material. In subsequent steps, the coated green bodies are dried and baked, whereby the fluxing substance is transformed to form a hard outer shell-like ferrite coating of calcium ferrite in those cases where the applied coating initially included lime or limestone.
U.S. Pat. No. 4,241,692 describes a device with a rotating drum with spray nozzles arranged inside for coating the surface of pellets or similar. The drum has an inlet and an outlet with essentially restricted openings and is equipped along its length with a number of spaces that are separated from one another by dividing walls projecting outwards within the drum where the said spaces are only joined with one another via apertures arranged in the said dividing walls.